Systems and methods for dedicated EGR cylinder exhaust gas temperature control

ABSTRACT

Systems and methods for increasing EGR gas temperature for an engine that includes at least one dedicated EGR cylinder. The dedicated EGR cylinder may provide exhaust gas to engine cylinders and the exhaust gas does not include exhaust gases from cylinders other than the dedicated EGR cylinder. The dedicated EGR cylinder may allow the engine to operate at higher EGR dilution levels.

FIELD

The present description relates to systems and methods for improvingoperation of an engine that operates with high levels of exhaust gasrecirculation (EGR). The methods may be particularly useful for enginesthat include one or more dedicated EGR (DEGR) cylinders that provideexternal EGR to engine cylinders.

BACKGROUND AND SUMMARY

An engine may be operated with one or more dedicated EGR cylinders(e.g., a cylinder that directs at least a portion of its exhaust flow,without exhaust from other cylinders, to provide external EGR to enginecylinders) that direct all of their exhaust gas to the intake air ofengine cylinders as external exhaust gas recirculation (EGR). Thisarrangement may allow the engine to operate with higher levels ofexhaust gas dilution. Consequently, engine pumping work may be reducedand engine efficiency may be improved. The external EGR may also bedirected through a cooler to reduce gas temperatures in the engine'scylinders, thereby making the engine less knock limited and furtherreducing NOx emissions. However, condensation may form within the EGRcooler, and the condensation may eventually be drawn into the enginewhere it may increase the possibility of engine misfire.

The inventors herein have recognized the above-mentioned disadvantagesof operating a highly diluted engine and have developed a method for anengine, comprising: recirculating exhaust gas from only a dedicated EGRcylinder to an intake of engine cylinders; increasing exhaust gastemperature in the dedicated EGR cylinder via increasing an air-fuelratio of the dedicated EGR cylinder in response to EGR coolercondensation; and operating remaining engine cylinders about astoichiometric air-fuel ratio in response to EGR cooler condensation.

By increasing an air-fuel ratio of a cylinder that is operating rich toimprove engine combustion stability of an engine that includes adedicated EGR (DEGR) cylinder, it may be possible to increase EGR gastemperature and removed condensation from an EGR cooler. A DEGR cylindermay typically be operated at a rich air-fuel ratio to improve combustionstability in an engine's non-DEGR cylinders. However, if higher EGR gastemperature is desired based on EGR cooler condensation, a cool engine,and/or low engine speed and load, EGR gas temperature may be increasedvia increasing or making less rich an air-fuel mixture supplied to aDEGR cylinder. In addition, when the air-fuel ratio of the DEGR cylinderis increased, resulting in less fuel in the recirculated exhaust gas,the amount of fuel injected to each of the remaining cylinders may beincreased to maintain a stoichiometric air-fuel ratio in the remainingcylinders. Additionally, in some examples, spark timing of the DEGRcylinder may be retarded as compared with spark timing of non-DEGRcylinders. In this way, EGR gas temperature may be increased withoutincreasing exhaust temperature of non-DEGR cylinders.

The present description may provide several advantages. In particular,the approach may reduce the possibility of engine misfire by allowingEGR cooler condensation to be reduced. Further, the approach may providefaster engine warm-up, thereby reducing engine emissions. Further, theapproach may improve engine efficiency at light engine loads.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings, where:

FIG. 1 is a schematic diagram of an engine;

FIGS. 2-3 show example engine variations that include a DEGR cylinder;

FIG. 4 is an example engine operating sequence according to the methodof FIG. 5.

FIG. 5 shows an example method for operating an engine that includes aDEGR cylinder.

DETAILED DESCRIPTION

The present description is related to operating an engine with highlydiluted cylinder mixtures. The engine cylinder mixtures may be dilutedusing recirculated exhaust gases that are byproducts of combustingair-fuel mixtures. The recirculated exhaust gases may be referred to asEGR. FIGS. 1-3 show example engine configurations that may be operatedat higher cylinder charge dilution levels to improve engine efficiencyand emissions. The engine may operate as shown in the sequence of FIG. 4according to the method shown in FIG. 5.

Referring to FIG. 1, internal combustion engine 10, comprising aplurality of cylinders as shown in FIGS. 2 and 3, one cylinder of whichis shown in FIG. 1, is controlled by electronic engine controller 12.Engine 10 includes combustion chamber 30 and cylinder walls 32 withpiston 36 positioned therein and connected to crankshaft 40. Combustionchamber 30 is shown communicating with intake manifold 44 and exhaustmanifold 48 via respective intake valve 52 and exhaust valve 54. Eachintake and exhaust valve may be operated independently with respect tovalves of other cylinders via intake cam 51 and an exhaust cam 53.Intake valve adjuster 85 advances or retards the phase of intake valve52 relative to a position of crankshaft 40. Additionally, intake valveadjuster 85 may increase or decrease an intake valve lift amount.Exhaust valve adjuster 83 advances or retards the phase of exhaust valve54 relative to a position of crankshaft 40. Further, exhaust valveadjuster 83 may increase or decrease an exhaust valve lift amount.

The position of intake cam 51 may be determined by intake cam sensor 55.The position of exhaust cam 53 may be determined by exhaust cam sensor57. In cases where combustion chamber 30 is part of a DEGR cylinder, thetiming and/or lift amount of valves 52 and 54 may be adjustedindependently of valves in other engine cylinders so that the cylinderair charge of the DEGR cylinder may be increased or decreased relativeto cylinder air charge of other engine cylinders during a same enginecycle. In this way, external EGR supplied to engine cylinders may exceedtwenty five percent of the cylinder charge mass for a four cylinderengine including a sole DEGR cylinder. Further, the internal EGR amountsof cylinders other than the DEGR cylinder may be adjusted independentlyof the DEGR cylinder by adjusting valve timing of those respectivecylinders. In some examples, an engine may include two or more DEGRcylinders. For example, an eight cylinder engine may include a DEGRcylinder for each cylinder bank.

Fuel injector 66 is shown positioned to inject fuel directly intocylinder 30, which is known to those skilled in the art as directinjection. Alternatively, fuel may be injected to an intake port, whichis known to those skilled in the art as port injection. Intake manifold44 is shown communicating with optional electronic throttle 62 whichadjusts a position of throttle plate 64 to control air flow from boostchamber 46 to intake manifold 44. In some examples, throttle 62 andthrottle plate 64 may be positioned between intake valve 52 and intakemanifold 44 such that throttle 62 is a port throttle. Compressor 162supplies air from air intake 42 to boost chamber 46. Compressor 162 isdriven by shaft 161 which is mechanically coupled to turbine 164.Compressor recirculation valve 158 may be selectively operated to reduceboost pressure. Waste gate 72 may be selectively opened and closed tocontrol the speed of turbine 164.

Driver demand torque may be determined from a position of acceleratorpedal 130 as sensed by accelerator pedal sensor 134. A voltage orcurrent indicative of driver demand torque is output from acceleratorpedal sensor 134 when driver's foot 132 operates accelerator pedal 130.

Distributorless ignition system 88 provides an ignition spark tocombustion chamber 30 via spark plug 92 in response to controller 12.Universal Exhaust Gas Oxygen (UEGO) sensor 126 is shown coupled toexhaust manifold 48 upstream of turbine 164 and catalytic converter 70.Alternatively, a two-state exhaust gas oxygen sensor may be substitutedfor UEGO sensor 126. Converter 70 can include multiple catalyst bricks,in one example. In another example, multiple emission control devices,each with multiple bricks, can be used. Converter 70 can be a three-waytype catalyst in one example.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-only(non-transitory) memory 106, random access memory 108, keep alive memory110, and a conventional data bus. Controller 12 is shown receivingvarious signals from sensors coupled to engine 10, in addition to thosesignals previously discussed, including: engine coolant temperature(ECT) from temperature sensor 112 coupled to cooling sleeve 114; ameasurement of engine manifold pressure (MAP) from pressure sensor 121coupled to intake manifold 44; a measurement of boost pressure frompressure sensor 122; an engine position sensor from a Hall effect sensor118 sensing crankshaft 40 position; a measurement of air mass enteringthe engine from sensor 120; and a measurement of throttle position fromsensor 58. Barometric pressure may also be sensed (sensor not shown) forprocessing by controller 12.

During operation, each cylinder within engine 10 typically undergoes afour stroke cycle: the cycle includes the intake stroke, compressionstroke, expansion stroke, and exhaust stroke. During the intake stroke,generally, the exhaust valve 54 closes and intake valve 52 opens. Air isintroduced into combustion chamber 30 via intake manifold 44, and piston36 moves to the bottom of the cylinder so as to increase the volumewithin combustion chamber 30. The position at which piston 36 is nearthe bottom of the cylinder and at the end of its stroke (e.g. whencombustion chamber 30 is at its largest volume) is typically referred toby those of skill in the art as bottom dead center (BDC). During thecompression stroke, intake valve 52 and exhaust valve 54 are closed.Piston 36 moves toward the cylinder head so as to compress the airwithin combustion chamber 30. The point at which piston 36 is at the endof its stroke and closest to the cylinder head (e.g. when combustionchamber 30 is at its smallest volume) is typically referred to by thoseof skill in the art as top dead center (TDC).

In a process hereinafter referred to as injection, fuel is introducedinto the combustion chamber. In a process hereinafter referred to asignition, the injected fuel is ignited by known ignition means such asspark plug 92, resulting in combustion. During the expansion stroke, theexpanding gases push piston 36 back to BDC. Crankshaft 40 convertspiston movement into a rotational torque of the rotary shaft. Finally,during the exhaust stroke, the exhaust valve 54 opens to release thecombusted air-fuel mixture to exhaust manifold 48 and the piston returnsto TDC. Note that the above is shown merely as an example, and thatintake and exhaust valve opening and/or closing timings may vary, suchas to provide positive or negative intake and exhaust valve openingoverlap, late intake valve closing, or various other examples.

FIG. 2 shows a schematic of a first example of engine 10 showingcylinders 1-4, one of which includes combustion chamber 30 of FIG. 1.The example engine configuration of FIG. 2 may include the devices shownin FIG. 1 for each engine cylinder. The intake valves 52 and exhaustvalves 54 of each cylinder may be opened and closed independently ofvalves of other engine cylinders via intake valve adjuster 85 andexhaust valve adjuster 83. For example, intake 52 and exhaust 54 valvesof cylinder number four may be opened and closed at different timingsrelative to engine crankshaft 40 and other engine cylinder valves. Thus,intake valve 52 of cylinder number four may be closed twenty five enginerotation degrees after BDC intake stroke of cylinder number four. On theother hand, during the same engine cycle, the intake valve of cylindernumber one may close five degrees after BDC intake stroke of cylindernumber one. Further, the engine configuration of FIG. 2 may be operatedaccording to the method of FIG. 5.

Throttle 62 regulates air flow into intake manifold 44, and intakemanifold 44 supplies air to each of cylinders 1-4. Intake valves 52 andexhaust valves 54 of one cylinder in the group of cylinders 1-4 may beoperated at different timings with respect to valve timing of othercylinders in the group of cylinders 1-4. In one example, valves ofcylinders 1-3 operate at a same timing, but valves of cylinder 4 operateat a different timing and/or same timing as valves for cylinders 1-3.For example, the intake valves of a cylinder in the group of cylinders1-3 may close at 10 crankshaft degrees after bottom dead center intakestroke of the cylinder where the intake valve is closing for aparticular engine cycle. On the other hand, the intake valves ofcylinder 4 may close at 20 crankshaft degrees after bottom dead centerintake stroke of cylinder 4 for the same engine cycle. In some examples,an air charge cooler (not shown) may be placed between compressor 162and throttle 62.

Exhaust from cylinders 1-3 is directed to exhaust manifold 48 beforebeing processed by a catalyst. Exhaust from cylinder 4 is routed tointake manifold 44 via DEGR cylinder bypass valve 205 and passage 209,or alternatively, to exhaust manifold 48 via DEGR cylinder bypass valve205 and passage 206. In some examples, DEGR cylinder bypass valve 205and passage 206 may be omitted. Exhaust gas from cylinder 4 may berouted from passage 209 to EGR cooler 212 or to EGR cooler bypasspassage 215 via EGR cooler bypass valve 211. EGR cooler 212 may reduce atemperature of EGR from the DEGR cylinder during middle to higher engineload conditions to reduce the possibility of engine knocking.

Each of cylinders 1-4 may include internal EGR via trapping exhaustgases from a combustion event in the respective cylinder and allowingthe exhaust gases to remain in the respective cylinder during asubsequent combustion event. The amount of internal EGR may be variedvia adjusting intake and exhaust valve opening and/or closing times, forexample by adjusting the amount of valve overlap (e.g., a duration whereintake and exhaust valves of a cylinder are simultaneously open). Byincreasing intake and exhaust valve opening overlap, additional EGR maybe retained in the cylinder during a subsequent combustion event whenexhaust manifold pressure is higher than intake manifold pressure.Further, decreasing intake valve and exhaust valve overlap may decreasecylinder charge temperatures and increase cylinder exhaust gastemperatures. Increasing intake valve and exhaust valve overlap mayincrease cylinder charge temperature and decrease cylinder exhaust gastemperature. In this example, external EGR is provided to cylinders 1-4solely via cylinder 4 exhaust and passage 209. External EGR is notprovided by exhaust flow from cylinders 1-3. Thus, in this example,cylinder 4 is the sole source of external EGR for engine 10. However, inV configuration engines a cylinder on each cylinder bank may be adedicated EGR cylinder. Passage 209 enters intake manifold 44 downstreamof compressor 162. Exhaust gases from cylinders 1-3 rotate turbine 164,and exhaust gases from cylinder 4 may selectively rotate turbine 164depending on an operating state of DEGR cylinder bypass valve 205.

Referring now to FIG. 3, a schematic of a second example of engine 10showing cylinders 1-4 is shown. One of cylinders 1-4 includes combustionchamber 30 of FIG. 1, and the remaining cylinders may include similardevices. The example engine configuration of FIG. 3 may include thedevices shown in FIG. 1 for each engine cylinder. The engine systemshown in FIG. 3 also includes many of the same devices and componentsdescribed in FIG. 2. Therefore, for the sake of brevity, the descriptionof like devices and components is omitted. However, the devices andcomponents operate and perform as described in FIG. 2.

In the example of FIG. 3, engine 10 includes passage 309 which does notcommunicate with exhaust manifold 48 and enters the engine upstream ofcompressor 162. EGR flow through passage 309 is adjusted via varyingtiming of intake valve adjuster 85 and exhaust valve adjuster 83 ofcylinder 4. For example, EGR flow from cylinder 4 to cylinders 1-4 maybe reduced via retarding intake valve closing (IVC) after BDC intakestroke cylinder 4. EGR flow from cylinder 4 to cylinders 1-4 may beincreased via advancing IVC toward BDC intake stroke cylinder 4.Further, EGR flow from cylinder 4 to cylinders 1-4 may be reduced viaretarding exhaust valve closing (EVC) from TDC exhaust stroke. EGR flowfrom cylinder 4 to cylinders 1-4 may be increased via advancing exhaustvalve closing (EVC) toward TDC exhaust stroke. Exhaust gas from cylinder4 may be routed from passage 309 to EGR cooler 312 or to EGR coolerbypass passage 315 via EGR cooler bypass valve 311. EGR cooler 312 mayreduce a temperature of EGR from the DEGR cylinder during middle tohigher engine load conditions to reduce the possibility of engineknocking.

Passage 309 may improve the possibility of increasing blow through(e.g., where contents of the intake manifold, such as air, is pushedthrough the cylinder while intake and exhaust valves of the cylinder aresimultaneously open during a cylinder cycle) for cylinder 4 when intakemanifold pressure is higher than pressure upstream of compressor 162.Rather than being routed downstream of compressor 162, exhaust gases arerouted upstream of compressor 162 where they may be exposed to pressurethat is lower than intake manifold pressure. EGR that flows throughpassage 309 enters intake manifold 44 after it is compressed viacompressor 162.

Thus, the system of FIGS. 1-3 provides for a vehicle system, comprising:an engine; a first variable valve adjustment device coupled to theengine and operating valves of a first cylinder; a second variable valveadjustment device coupled to the engine and operating valves of a secondcylinder; a passage fluidly coupling the exhaust side of the firstcylinder to an air intake of the engine, the passage not fluidlycoupling the exhaust side of other engine cylinders to the air intake;and a controller including non-transitory instructions for transitioningfrom combusting a rich air-fuel mixture in the first cylinder tocombusting a stoichiometric air-fuel ratio in the first cylinder inresponse to a request to increase a temperature of EGR gas.

The system of FIGS. 1-3 also provides for additional instructions formaintaining a stoichiometric air-fuel ratio in the second cylinder. Thevehicle system further comprises an exhaust passage leading from thesecond cylinder to atmosphere, the exhaust passage not in fluidiccommunication with the first cylinder. The vehicle system includes wherethe passage enters the air intake upstream of a throttle and compressor.The vehicle system further comprises an EGR cooler and additionalinstructions to increase exhaust gas temperature exiting the firstcylinder without additional instructions to increase exhaust gastemperature exiting the second cylinder. The vehicle system furthercomprises instructions to bypass exhaust gases from the first cylinderaround the EGR cooler.

Referring now to FIG. 4, an example engine operating sequence accordingto the method of FIG. 5 and the system of FIGS. 1-3 is shown. Thesequence of FIG. 4 is merely one example simulated sequence that may beprovided by the method of FIG. 5.

The first plot from the top of FIG. 4 is a plot of driver demand torqueversus time. The Y axis represents driver demand torque and driverdemand torque increases in the direction of the Y axis arrow. The X axisrepresents time and time increases from the left side of FIG. 4 to theright side of FIG. 4. Driver demand torque may be determined fromaccelerator pedal position.

The second plot from the top of FIG. 4 is a plot of engine temperatureversus time. The Y axis represents engine temperature and enginetemperature increases in the direction of the Y axis arrow. The X axisrepresents time and time increases from the left side of FIG. 4 to theright side of FIG. 4. Horizontal line 401 represents a threshold enginetemperature where the engine is determined to be warm.

The third plot from the top of FIG. 4 is a plot of engine intake andexhaust valve overlap (e.g., when intake valves and exhaust valves aresimultaneously open) for the DEGR cylinder. Valve overlap increases inthe direction of the Y axis arrow and is zero at the X axis level. The Xaxis represents time and time increases from the left side of FIG. 4 tothe right side of FIG. 4.

The fourth plot from the top of FIG. 4 is a plot of cylinder air-fuelratio for non-DEGR cylinders versus time. The Y axis represents cylinderair-fuel ratio for non-DEGR cylinders. Horizontal line 402 represents astoichiometric air-fuel ratio. The engine cylinders are injected with alean mixture when the trace is above horizontal line 402, and the enginecylinders are injected with a rich mixture when the trace is belowhorizontal line 402. The X axis represents time and time increases fromthe left side of FIG. 4 to the right side of FIG. 4.

The fifth plot from the top of FIG. 4 is a plot of cylinder air-fuelratio for a DEGR cylinder versus time. The Y axis represents cylinderair-fuel ratio for the DEGR cylinder. Horizontal line 404 represents astoichiometric air-fuel ratio. The DEGR cylinder is operating with alean mixture when the trace is above horizontal line 404, and the DEGRengine cylinder is operating with a rich mixture when the trace is belowhorizontal line 404. The X axis represents time and time increases fromthe left side of FIG. 4 to the right side of FIG. 4.

The sixth plot from the top of FIG. 4 is a plot of DEGR cylinder sparktiming and non-DEGR spark timing versus time. The Y axis representscylinder spark timing and cylinder spark timing advances in thedirection of the Y axis arrow. The X axis represents time and timeincreases from the left side of FIG. 4 to the right side of FIG. 4.Dashed line 406 represents spark timing for non-DEGR cylinders. Solidline 408 represents spark timing for DEGR cylinders. Spark timing fornon-DEGR cylinders matched spark timing for DEGR cylinder when onlysolid line 408 is visible.

The seventh plot from the top of FIG. 4 is a plot of a variable thatindicates the presence or absence of EGR cooler condensation versustime. The Y axis represents a level of the variable that indicates EGRcooler condensation. EGR cooler condensation is indicated when the traceis at a higher level near the Y axis arrow level. The X axis representstime and time increases from the left side of FIG. 4 to the right sideof FIG. 4.

At time T0, the engine is not operating and the driver demand torque iszero. The driver demand torque includes any input by the driver to theaccelerator pedal as well as any internal controller requested torquesuch as torque to operate the engine at an elevated idle speed. Theengine temperature is low and the exhaust valve overlap is at a middlelevel. No EGR cooler condensation is indicated.

Between time T0 and time T1, the engine is started and the driver demandtorque is increased to operate the engine at a higher idle speed. Byoperating the engine at a higher idle speed, the engine may be warmedmore quickly. The engine temperature begins to increase in response tostarting the engine. The non-DEGR cylinders are operated lean andtransition to stoichiometric operation. The DEGR cylinder is operatednear a stoichiometric air-fuel ratio (e.g., 0.5 air-fuel ratio rich ofstoichiometric conditions). By operating the DEGR cylinder nearstoichiometric conditions, EGR temperature may be increased whilecombustion stability may be increased as compared to if the DEGRcylinder where operating lean. The non-DEGR cylinders are operated leanto decrease hydrocarbons in the exhaust gases. The DEGR cylinder and thenon-DEGR cylinders are operated at a retarded spark timing to increaseengine temperature and EGR temperature. The DEGR cylinder's spark timingis more retarded than the non-DEGR cylinders and the spark timingadvances as engine temperature increases. The DEGR cylinder intake andexhaust overlap is decreased after the DEGR air-fuel ratio isstoichiometric so that EGR temperature may be further increased. Thereis no indication of EGR cooler condensation. Driver demand torquefollows driver demand. In examples where the system includes a bypassvalve (e.g., 205 of FIG. 2), the bypass valve is open while spark timingis retarded.

At time T1, engine temperature reaches threshold temperature 401 and theintake and exhaust valve overlap is increased in response to the enginereaching a desired temperature. Further, the DEGR cylinder's air-fuelratio is richened to improve engine combustion stability. DEGR andnon-DEGR cylinder spark timing advances as the engine temperatureincreases and there is no indication of EGR cooler condensation. Driverdemand torque follows driver demand and the non-DEGR cylinders operatewith a stoichiometric air-fuel ratio.

Between time T1 and time T2, driver demand torque follows driver demandand engine temperature remains warm. DEGR cylinder intake and exhaustvalve overlap remains longer in duration and the DEGR cylinder operatesrich of stoichiometry to improve engine combustion stability as theengine operates at higher EGR dilution levels. EGR cooler condensationis not indicated. Near time T2 the driver demand torque is reducedallowing the exhaust gas temperatures to be reduced.

At time T2, EGR cooler condensation is indicated by the EGR coolercondensation variable transitioning to a higher level. EGR coolercondensation may be determined via a sensor (e.g., a humidity sensor) orinferred from engine operating conditions. Condensation may increase theprobability of engine misfire by inducting water droplets and/or watervapor into the engine from the EGR cooler. The engine is operating atidle conditions where there is an opportunity to retard spark timingwithout the driver perceiving a loss of engine power to increase exhaustgas temperature from the DEGR cylinder so that condensation may bepurged from the EGR cooler. Therefore, spark timing in the DEGR cylinderis retarded further to increase the temperature of exhaust gases exitingthe DEGR cylinder and entering the EGR cooler.

The warmer EGR gases begin to vaporize any water or condensation in theEGR cooler. If the system is equipped with an EGR cooler bypass valve(e.g., 211 or 311), the valve may be positioned to allow EGR to flowthrough the EGR cooler when condensation is present in the EGR coolerand positioned to allow EGR to flow around the EGR cooler when enginetemperature is less than a threshold temperature. Spark timing in thenon-DEGR cylinders remains at a more advanced timing as compared tospark timing for the DEGR cylinder. The intake and exhaust valve overlapis decreased to further increase exhaust gas temperature in the DEGRcylinder. In some examples, the exhaust valve opening (EVO) time may beadvanced to increase temperature of exhaust gases exiting the DEGRcylinder instead of decreasing intake and exhaust valve overlap. TheDEGR cylinder continues to operate with a rich air-fuel mixture so thatcombustion stability may be improved in the non-DEGR cylinders. Thenon-DEGR cylinders are fueled slightly lean of stoichiometry to providean overall stoichiometric mixture in non-DEGR cylinders. Enginetemperature remains at a warmer level.

At time T3, the EGR cooler condensation indication variable changesstate to a lower level to indicate the absence of condensation in theEGR cooler. The DEGR cylinder spark timing is advanced and the intakeand exhaust valve overlap is increased in response to the absence ofindicated EGR condensation. The DEGR cylinder continues to operate witha rich air-fuel mixture and the non-DEGR cylinders continue to operateslightly lean of stoichiometry.

In this way, temperature of exhaust gases exiting the DEGR cylinder maybe increased to warm the engine and reduce EGR cooler condensationwithout simultaneously taking action in non-DEGR cylinders to increasetheir exhaust gas temperatures. Further, exhaust gas temperature may beincreased in the DEGR cylinder during low engine load conditions, suchas idle, to increase engine fuel economy by decreasing engine pumpingwork.

Referring now to FIG. 5, a method for operating an engine that includesat least one DEGR cylinder is shown. The method of FIG. 5 may be storedas executable instructions in non-transitory memory of controller 12shown in FIG. 1. Further, the method of FIG. 5 may provide the operatingsequence shown in FIG. 4.

At 502, method 500 determines engine operating conditions. Engineoperating conditions may include but are not limited to engine speed,engine load, driver demand torque, EGR cooler condensation, and enginetemperature. Method 500 proceeds to 504 after engine operatingconditions are determined.

At 504, method 500 judges if the engine is being started at an enginetemperature less than (L.T.) a threshold temperature. The enginestarting period may go on from engine cranking until the engine reachesa threshold temperature or amount of time since engine stop. Enginetemperature may be determined via a temperature sensor and enginestarting may be determined based on engine speed. If method 500 judgesthat the engine is being started at a temperature less than a thresholdtemperature, the answer is yes and method 500 proceeds to 506.Otherwise, the answer is no and method 500 proceeds to 520.

At 506, method 500 retards spark timing from baseline spark timing(e.g., spark timing for a warmed up engine at similar engine speed andload). For example, if at engine idle and the engine is warm, sparktiming may be 15 degrees before top-dead-center (BTDC) compressionstroke. Spark timing may be adjusted to 5 degrees after top-dead-center(ATDC) compression stroke during a cold engine start (e.g., 18° C.). Byretarding spark timing engine heating may be increased and hydrocarbonemissions may be reduced. Further, if the system includes a bypass valve(e.g., 205 of FIG. 2) the bypass valve may be opened to allow exhaustfrom the DEGR cylinder to flow to atmosphere instead of being directedto the engine intake. The bypass valve is closed if engine temperatureis greater than the threshold temperature. Method 500 proceeds to 508after spark timing is retarded from baseline spark timing.

At 508, method 500 operates non-DEGR cylinders lean or rich ofstoichiometry. If the engine is being started at an elevated temperaturethat is less than warm engine operating temperature (e.g., greater than40° C.) and catalyst temperature is warm, the engine may be operatedslightly rich to reduce NOx emissions. If the engine is cool started(e.g., at a temperature less than 40° C., but greater than 10° C.), thenon-DEGR cylinders may be operated lean of stoichiometry to reducehydrocarbon emissions. Method 500 proceeds to 510 after non-DEGRcylinder air-fuel ratios are established and output.

At 510, method 500 operates DEGR cylinders substantially atstoichiometry (e.g., within ±3 percent). By operating the DEGR cylindersat stoichiometry, exhaust gas temperature from the DEGR cylinders mayincrease. Method 500 proceeds to 512 after DEGR cylinders are operatedat stoichiometry.

At 512, method 500 advances exhaust valve opening time and/or decreasesintake and exhaust valve overlap for the DEGR cylinders. By opening theexhaust valves early, less expansion work is performed by the exhaustgases, thereby allowing the exhaust gases to exit the DEGR cylinders athigher temperatures. Decreasing the intake and exhaust valve overlapdecreases internal EGR within the DEGR cylinder and increases exhaustgas temperature via reducing cylinder charge dilution. Method 500proceeds to exit after EVO and/or overlap are adjusted.

Thus, during cool engine starting conditions, DEGR cylinder spark timingmay be retarded to provide more retarded spark timing than non-DEGRcylinders and higher exhaust gas temperatures. Additionally, DEGRcylinders may be operated lean to increase exhaust gas temperatures.Further, valve timing may be adjusted to increase temperature of exhaustgases flowing from DEGR cylinders to increase EGR temperature. Thehigher EGR temperature may help to warm the engine faster.

At 520, method 500 judges if there is EGR condensation. EGR condensationmay be determined via a sensor (e.g., humidity sensor) or via inference.If method 500 judges that condensation is present, the answer is yes andmethod 500 proceeds to 540. Otherwise, the answer is no and method 500proceeds to 550.

Alternatively, if the system includes a water-gas shift catalyst insteadof an EGR cooler, method 500 judges if there is condensation at thewater-gas shift catalyst via a sensor or inference. If method 500 judgesthat condensation is present, the answer is yes and method 500 proceedsto 540. Otherwise, the answer is no and method 500 proceeds to 550

At 550, method 500 operates DEGR cylinders and non-DEGR cylinders atbase air-fuel ratios, spark timing, and valve timings. The DEGR cylinderair-fuel ratio may be rich and the non-DEGR cylinder air-fuel ratio maybe stoichiometric at baseline conditions. The DEGR and non-DEGR cylinderspark timings may be the same at baseline conditions. The DEGR andnon-DEGR cylinder may have the same or different valve timings atbaseline conditions. Method 500 proceeds to exit after air-fuel ratio,spark timing, and valve timing are adjusted.

At 540, method 500 judges if engine load is less than (L.T.) a thresholdengine load. Additionally, method 500 may judge if engine speed is lessthan a threshold speed. If method 500 judges that engine load and/orspeed are less than threshold values, the answer is yes and method 500proceeds to 542. Otherwise, the answer is no and method 500 proceeds to522.

At 522, method 500 operates DEGR cylinders near stoichiometric air-fuelratio (e.g., from 0.5 air-fuel ratio rich of stoichiometry tostoichiometric conditions) in response to condensation in the EGR cooleror water-gas catalyst. By operating the DEGR cylinders near astoichiometric air-fuel ratio, temperature of exhaust gases flowing fromthe DEGR cylinder to the EGR cooler or water-gas catalyst may beincreased so as to vaporize condensed water in the EGR cooler orwater-gas catalyst. The vaporized water may then be inducted into theengine at a rate that lowers the possibility of engine misfire. Fuelsupplied to the non-DEGR cylinders may be adjusted to maintain non-DEGRcylinders at stoichiometric conditions. For example, if the DEGRcylinder is adjusted from a rich air-fuel mixture to a lean air-fuelmixture, fuel supplied to the non-DEGR cylinders may be increased so asto maintain the DEGR cylinders at stoichiometric conditions. Method 500proceeds to 524 after beginning to operate the DEGR cylinders with astoichiometric air-fuel ratio.

At 524, method 500 judges if EGR temperature entering the EGR cooler orwater-gas catalyst is greater than (G.T.) a threshold temperature (e.g.,100° C.) within a threshold amount of time. The EGR temperature may beinferred or sensed. If method 500 judges that EGR temperature is greaterthan the threshold temperature in an allotted amount of time, the answeris yes and method 500 proceeds to 528. Otherwise, the answer is no andmethod 500 proceeds to 526.

At 526, method 500 advances exhaust valve opening time and/or decreasesintake and exhaust valve overlap for the DEGR cylinders. By opening theexhaust valves early, less expansion work is performed by the exhaustgases, thereby allowing the exhaust gases to exit the DEGR cylinders athigher temperatures. Decreasing the intake and exhaust valve overlapdecreases internal EGR within the DEGR cylinder and increases exhaustgas temperature via reducing cylinder charge dilution. Valve timing ofnon-DEGR cylinders may not be adjusted in response to increasing EGR gastemperature. Method 500 proceeds to 528 after EVO and/or overlap areadjusted.

At 528, method 500 judges if EGR condensation is removed. In oneexample, method 500 may judge EGR condensation is removed based onoutput of a sensor. In other examples, EGR condensation amount may beinferred. If method 500 judges that EGR condensation is removed, theanswer is yes and method 500 proceeds to 530. Otherwise, the answer isno and method 500 returns to 528.

At 530, method 500 increases intake and exhaust valve overlap, retardsEVO timing, and/or operates DEGR cylinders with a rich air-fuel ratio.In other words, method 500 returns valve timing and air-fuel ratio ofDEGR cylinders back to baseline values. The DEGR cylinder is operatedrich to provide hydrogen to non-DEGR cylinders, thereby improvingcombustion stability at higher EGR flow rates. Method 500 proceeds toexit after DEGR cylinder air-fuel ratio and valve timings are returnedto base values.

In this way, DEGR cylinder air-fuel ratio and valve timing may beadjusted to increase temperature of exhaust gases exiting the DEGRcylinder so that EGR cooler or water-gas catalyst condensation may bereduced. If the engine is not operating at a middle or higher engineload, DEGR cylinder exhaust gas temperature may be increased viaretarding spark. However, at higher engine loads, method 500 does notretard spark timing in response to EGR condensation to increase EGRtemperature. The engine load, DEGR cylinder air-fuel ratio, and valvetiming may be sufficient to produce higher exhaust temperatures from theDEGR cylinder. Thus, engine performance may not be reduced to increaseexhaust gas temperature via spark retard.

At 542, method 500 operates non-DEGR cylinders with a stoichiometricair-fuel ratio. The non-DEGR cylinders may be operated about astoichiometric air-fuel ratio (e.g., within ±0.5 air-fuel ratio of astoichiometric air-fuel ratio) to improve catalyst efficiency. Method500 proceeds to 544 after non-DEGR cylinder air-fuel ratio is adjusted.

At 544, method 500 operates DEGR cylinders at an air-fuel ratio that isrich of stoichiometry. The DEGR cylinder is operated rich to providehydrogen to the non-DEGR cylinders which may improve combustionstability when the engine is operating at light loads with a relativelyhigh EGR concentration. Method 500 proceeds to 546 after adjusting theair-fuel ratio of the DEGR cylinder.

At 546, method 500 increases spark retard of the DEGR cylinder. Byincreasing spark retard, combustion in the cylinder is delayed so thatcylinder's air-fuel mixture oxidizes later in the combustion cycle,thereby reducing mechanical work produced by the DEGR cylinder andincreasing exhaust gas temperature. The higher exhaust gas temperatureallows the engine throttle to be opened further without increasingengine torque since the warmed EGR displaces additional air from theengine cylinders. Consequently, engine pumping work may be reduced andengine efficiency may be increased. Spark timing of non-DEGR cylindersmay remain at baseline spark timing. Method 500 proceeds to 548 afterspark timing is adjusted.

At 548, method 500 advances exhaust valve opening time and/or decreasesintake and exhaust valve overlap for the DEGR cylinders. By opening theexhaust valves early, less expansion work is performed by the exhaustgases, thereby allowing the exhaust gases to exit the DEGR cylinders athigher temperatures. Decreasing the intake and exhaust valve overlapdecreases internal EGR within the DEGR cylinder and increases exhaustgas temperature via reducing cylinder charge dilution. Method 500proceeds to exit after EVO and/or overlap are adjusted.

In this way, air-fuel ratios of DEGR and non-DEGR cylinders may beadjusted to increase EGR gas temperature for improving engine efficiencyand emissions. Further, valve timing and spark timing may be adjusted toincrease EGR temperature. Further still, adjustments to valve timing ofnon-DEGR cylinders may not be adjusted so as to maintain exhaust gastemperature and combustion properties of non-DEGR cylinders when valvetiming of DEGR cylinders is adjusted to increase EGR gas temperature.Additionally, the cylinder air amount of the DEGR cylinder may beincreased when spark timing of the DEGR cylinder is retarded to maintaintorque supplied by the DEGR cylinder.

Thus, the method of FIG. 5 provides for a method for an engine,comprising: recirculating exhaust gas from only a dedicated EGR cylinderto an intake of engine cylinders; increasing EGR temperature in thededicated EGR cylinder via increasing an air-fuel ratio of the dedicatedEGR cylinder in response to EGR cooler condensation; and decreasing anair-fuel ratio of the remaining engine cylinders (e.g., non-dedicatedEGR cylinders) in response to the EGR cooler condensation. The methodfurther comprises advancing exhaust valve opening timing of thededicated EGR cylinder without advancing exhaust valve opening timing ofthe remaining engine cylinders. The method further comprises decreasingintake and exhaust valve opening overlap of the dedicated EGR cylinderwithout decreasing intake and exhaust opening overlap of the remainingengine cylinders.

In some examples, the method includes where the dedicated EGR cylinderprovides exhaust gas to the cylinders upstream of a compressor. Themethod includes where the dedicated EGR cylinder provides exhaust gas tothe remaining cylinders downstream of the compressor. The method alsoincludes where the air-fuel ratio of the dedicated EGR cylinder is madeleaner via increasing the air-fuel ratio of the dedicated EGR cylinder,and where the air-fuel ratio of the remaining engine cylinders (e.g.,non-dedicated EGR cylinders) is made richer via decreasing the air-fuelratio of remaining engine cylinders.

The method of FIG. 5 also provides for a method for an engine,comprising: recirculating exhaust gas from only a subset of enginecylinders to an intake of engine cylinders; increasing EGR gastemperature by first retarding spark timing and then by adjusting valvetiming during a first mode; and increasing EGR gas temperature by firstincreasing an air-fuel ratio of the subset of engine cylinders and thenby adjusting valve timing of the subset of engine cylinders during asecond mode, the second mode different than the first mode. The methodincludes where the first mode is during an engine start when the engineis not fully warm. The method includes where the second mode is duringcondensation purging of an EGR cooler.

In some examples, the method includes where adjusting valve timingincludes advancing exhaust valve opening time. The method also includeswhere adjusting valve timing includes increasing intake and exhaustvalve overlap. The method further comprises decreasing an air-fuel ratioof the remaining engine cylinders during the second mode. The methodincludes where an air-fuel ratio of the remaining engine cylinders isstoichiometric. The method includes where the subset of engine cylindersare operated with a stoichiometric air-fuel ratio.

As will be appreciated by one of ordinary skill in the art, methoddescribed in FIG. 5 may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various steps orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the objects, features, andadvantages described herein, but is provided for ease of illustrationand description. Although not explicitly illustrated, one of ordinaryskill in the art will recognize that one or more of the illustratedsteps or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described actions,operations, methods, and/or functions may graphically represent code tobe programmed into non-transitory memory of the computer readablestorage medium in the engine control system.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

The invention claimed is:
 1. A method for operating an engine withexhaust gas recirculation (EGR), comprising: recirculating exhaust gasfrom only a subset of engine cylinders to an intake of engine cylindersvia a controller; reducing EGR cooler condensate via increasing EGR gastemperature by first retarding spark timing via the controller and thenby adjusting valve timing via the controller during a first mode; andreducing EGR cooler condensate via increasing EGR gas temperature byfirst increasing an air-fuel ratio of the subset of engine cylinders viathe controller and then by adjusting valve timing of the subset ofengine cylinders during a second mode via the controller, the secondmode different than the first mode.
 2. The method of claim 1, where thefirst mode is during an engine start when the engine is not fully warm.3. The method of claim 1, where the second mode is during condensationpurging of an EGR cooler.
 4. The method of claim 1, where adjustingvalve timing includes advancing exhaust valve opening time.
 5. Themethod of claim 1, where adjusting valve timing includes increasingintake and exhaust valve overlap.
 6. The method of claim 1, furthercomprising decreasing an air-fuel ratio of engine cylinders other thanengine cylinders included in the subset of engine cylinders during thesecond mode.
 7. The method of claim 6, where the air-fuel ratio of theengine cylinders other than the engine cylinders included in the subsetof engine cylinders is stoichiometric.
 8. The method of claim 1, wherethe subset of engine cylinders is operated with a stoichiometricair-fuel ratio.
 9. A vehicle system, comprising: an engine including afuel injector; a first variable valve adjustment device coupled to theengine and operating valves of a first cylinder; a second variable valveadjustment device coupled to the engine and operating valves of a secondcylinder; a passage fluidly coupling an exhaust side of the firstcylinder to an air intake of the engine, the passage not fluidlycoupling an exhaust side of other engine cylinders to the air intake;and a controller including executable instructions stored innon-transitory memory for transitioning from combusting a rich air-fuelmixture in the first cylinder to combusting a less rich air-fuel ratioin the first cylinder via adjusting the fuel injector in response to arequest to increase a temperature of EGR gas.
 10. The vehicle system ofclaim 9, further comprising additional instructions for maintaining astoichiometric air-fuel ratio in the second cylinder.
 11. The vehiclesystem of claim 9, further comprising an exhaust passage leading fromthe second cylinder to atmosphere, the exhaust passage not in fluidiccommunication with the first cylinder.
 12. The vehicle system of claim9, where the passage enters the air intake upstream of a throttle and acompressor.
 13. The vehicle system of claim 9, further comprising an EGRcooler and additional instructions to increase exhaust gas temperatureexiting the first cylinder without additional instructions to increaseexhaust gas temperature exiting the second cylinder.
 14. The vehiclesystem of claim 13, further comprising instructions to bypass exhaustgases from the first cylinder around the EGR cooler.